identify users in posthog via user id

[ChristopherChudzicki]

What are the relevant tickets?

https://github.com/mitodl/hq/issues/5977

Description (What does it do?)

This PR enables identifying users in posthog via MIT Learn user ids. This lets us use, e.g., specific feature flags.

How can this be tested?

Here's one way you could test this.

1. Connect a personal PostHog account to MIT Learn by setting POSTHOG_PROJECT_API_KEY in env/shared.local.env
2. Create two users in your instance of Learn
   • I find it convenient to use separate Chrome profiles or separate browsers for different users.
3. In PostHog, create a feature flag named lr_drawer_v2. Set it to apply to 100% of users with a specific id. Something like:
   
4. Check that:
   • When logged in as that user, you see the new learning resource drawer (wider drawer—same as RC)
   • Now log out, still in same browser window. You should see old drawer (narrower drawer—same as Prod)
   • When logged in as a different user, you should see old drawer (narrower drawer, same as Prod)

[jkachel]

Tested and started seeing events come through with my user ID.

LGTM 👍